Current analogue to digital converters (ADC) do not have a sufficient dynamic range to process the full range of receive signals that the ADCs are likely to encounter in a mobile communications network radio base station. It is therefore known in the art to break down the likely range of receive signals into two or more sub-ranges of radio signals. Some form of gain control or gain switching is used to reduce the amplitude of the radio signal impinging upon the ADC in strong signal conditions. Each one of the switched receivers in the base station will have its own threshold level at which the gain of the receiver is switched (e. g. from “high” to “low” gain) and a (different) threshold level at which the gain changes in the reverse direction (i.e. from “low” to “high”). An element of hysteresis is deliberately build into the process of gain switching in order to prevent excessive amounts of gain control or gain switching from occurring when the receive signals are close to or at the threshold level.
FIG. 1 shows a prior art antenna array with a single downconversion stage. The receive signals are received at antenna elements Ant and passed to a duplex filter 100. The duplex filter 100 removes any unwanted out-off-band signals from the receive signals. The receive signals are passed through a gain switch 105. The gain switch 105 can either pass the receive signal unmodified or reduce the amplitude of the receive signal by adding attenuation to the receive signal. The gain switch 105 is connected by a gain switch control line 106 to an output of a digital signal processor 130. The amount of attenuation added to the receive signal is controlled by a signal along the gain switch control line 106 from the digital signal processor 130, as will be explained later.
The remainder of the receiver shown in FIG. 1 is a conventional single downconversion design with a low noise amplifier 200 receiving the receive signals (either unmodified or attenuated) from the gain switch 105 and passing the amplified (and possibly attenuated) receive signals to a mixer 110 which downconverts the amplified receive signals to a lower frequency. The downconverted receive signals are passed to a bandpass filter 120 to remove any unwanted out-off-band signals from the downconverted receive signals. The output of the bandpass filter 120 is connected to an analogue-to-digital converter 125 which converts the down converted signals in the analogue domain to the digital domain at a digital intermediate frequency. The digital signals are passed to the digital signal processor 130 and then output as baseband digital signals 135.
The ADC 125 has a limited dynamic range and the digital signal processor 130 will analyse the digital signals in order to determine if the ADC 125 is close to overloading or saturation. The digital signal processor 130 does this analysis by comparing the digital signal level output from the ADC 125 to a predetermined threshold level. If the threshold level is exceeded, the strength of the receive RF signals is reduced by switching in the attenuation in the gain switch 105. A typical six-sector radio base station will have six of these receivers (or twelve if diversity reception is applied). The presence of six identical receivers in the base station increases the amount of real estate required on a chip and in the base station for all six of the receivers and also requires sufficient processing power to ensure that all six of the receivers act in tandem with each other.